User Manual

IP Subnet Masks
6-5
All incoming SLIP users that do not use a custom site will use the default site for the connection. To require
that default site users use an IP address from the pool, use the Define Site Default IP Remoteaddress
command.
Figure 6-8: Using the Address Pool for the Default Site
6.1.2.2 Dialing Out to an ISP
An SCS site can be configured to dial out to an ISP that uses PPP, such as Earthlink. Most ISPs will want
to assign a nameserver and an IP address to the SCS. To accept this assignment, set the SCS IP address
assignment to dynamic and set its nameserver to 0.0.0.0.
Figure 6-9: Using the SCS With an ISP
These settings allow site irvine to accept an IP address and a nameserver setting from the ISP.
6.2 Subnet Masks
IP networks can be divided into several smaller networks by subnetting. When you request a connection,
the SCS decides whether the desired TCP/IP host is on the local network segment with the help of the
subnet mask. The mask identifies the network and node parts of the IP address, which is then applied to
the addresses of both the SCS and the remote host. If the resulting addresses are identical, the connection is
deemed local and the host is contacted directly. If not, the connection attempt and all subsequent messages
to this host will be directed to the SCS’s gateway host for forwarding. All hosts must agree on the subnet
mask for a given network.
For example, IP address 128.1.150.35 is on a class B network. The network portion of this address is 128.1.
This large network can be broken down into 254 networks using a subnet mask of 255.255.255.0, which
makes the network portion 128.1.150.
It is not always necessary to divide a network into subnetworks. To determine whether subnetting is
required, a number of factors should be considered, including the network size and whether or not network
traffic needs to be isolated in a particular area.
When you configure the IP address for the first time, a default subnet mask will be configured automatically.
This default subnet mask should work for most networks. If your network is divided into subnetworks, you
will need to create a custom subnet mask. To override the default subnet masks, use the Set/Define IP
Subnet Mask command.
Figure 6-10: Setting the Subnet Mask
It is also possible to learn a subnet mask from BOOTP, though not all BOOTP server implementations
support sending subnet masks. Check your BOOTP server’s documentation.
Local>> DEFINE SITE DEFAULT IP REMOTEADDRESS 192.0.1.100 192.0.1.105
Local>> DEFINE SITE irvine IP IPADDRESS DYNAMIC
Local>> DEFINE SERVER NAMESERVER 0.0.0.0
Local>> DEFINE IP SUBNET MASK 255.255.0.0